This invention relates to an image forming apparatus, such as a copying machine or a printer, and more particularly to an image forming apparatus which forms an image on a recording medium by using an electrophotographic process.
FIG. 1 shows a sectional view of an image forming apparatus using an electrophotographic process. In FIG. 1, a photosensitive drum 1 bears an image thereon and rotates in the clockwise direction. A discharge device 2, comprising a corona wire and grids, uniformly applies electrostatic charge to the photosensitive drum 1. A preconditioning exposure device 8 exposes the photosensitive drum 1 prior to the corona discharging by the discharge device 2 so as to make the surface potential of the photosensitive drum 1 at approximately zero voltage level. An optical system 3 projects the reflected light from an original document onto the photosensitive drum 1. A developing device 4 develops an electrostatic latent image on the photosensitive drum 1 with toner. A cleaning device 5 cleans residual toner on the photosensitive drum 1. A transfer charging device 7 transfers the toner image on the photosensitive drum 1 onto a recording medium P. A test image 11 is formed on the photosensitive drum 1. A light emitting diode (LED) 12 exposes the test image 11 and a photoelectric sensor 13 detects the amount of the light reflected from the test image 11. A density conversion circuit 6 converts the output voltage from the photoelectric sensor 13 into density values. A central processing unit (CPU) 10 controls various image forming conditions in accordance with the density values input from the density conversion circuit 6.
An image forming operation of the image forming apparatus shown in FIG. 1 will now be described.
The photosensitive drum 1 is uniformly charged by the discharging device 2. Next, the light image corresponding to the original image is exposed through the optical system 3 whereby the electrostatic latent image is formed on the photosensitive drum 1. The electrostatic latent image is developed by the developing device 4 with toner and becomes a visible image. Then, the toner image is transferred onto the recording medium P by the transfer charging device 7.
The density of the toner image thus transferred onto the recording medium P depends upon the various image forming conditions, for example, the surface potential of the photosensitive drum 1 applied by the discharge device 2, the density of toner in the developing device 4, the developing power of the developing device 4 and the transfer power of the transfer charging device 7. The density of the toner image in a laser beam printer, which exposes a light image on the photosensitive drum with a laser beam, also depends on the image signals for modulating the laser beam or the intensity of the laser beam.
Accordingly, it is necessary to control the various image forming conditions and monitor and if necessary adjust them so as to form an image with an optimum density. Therefore, prior to the image formation, the density of the test image 12 formed on the photosensitive drum 1 is detected by the photoelectric sensor 13, and the CPU 10 controls the various image forming conditions in accordance with the detected density of the test image 12. The above mentioned test image 12 is formed on the photosensitive drum 1 in accordance with the electrophotographic process by exposing a predetermined light image through the optical system 3.
FIG. 2 shows an example of the test image 12 formed on the photosensitive drum 1. A black image 14 where the toner adheres and a white image 15 where the toner does not adhere are formed within the detection area of the photoelectric sensor 13 on the photosensitive drum 1. By detecting the densities of the black image 14 and the white image 15 by means of the photoelectric sensor 13, the maximum black density and the contrast between the black and white images can be determined. The various image forming conditions can be controlled in accordance with the determined results so as to control the density of the image to be formed.
In the above described construction, the density of the test image 11 is detected by the single photoelectric sensor 13, so that the density of the simple test image formed within a predetermined small area on the photosensitive drum 1 can be detected.
However, the image formed on the photosensitive drum 1 on the basis of the original document is usually more complex than the test image 11, accordingly it is impossible to control the density of the complex original image to be formed.